Insofar as industrial and other blue collar workers are more likely to bring home toxic materials on their person, and also are more likely to smoke than those in other occupations, members of a household are more likely to be subject to paraoccupational exposure and belong to lower socioeconomic strata if the household contains a smoker than if the household does not contain a smoker.

Thus observed differences in risk of mortality or morbidity ascribed to ETS on the basis of a comparison of households with and without smokers may be partly or entirely due to differences in paraoccupational exposure and socioeconomic strata.

Similarly, differences in mortality and morbidity ascribed to paraoccupational exposure may be partly or entirely due to differences in ETS exposure that are also related to social class and to types of occupation.

Unfortunately, there are no data now in existence that could help determine separately the effects of these major confounded variables.

There exists, then, a situation in which two explanations are advanced for respiratory diseases among members of a household, each based on similar study populations but focused on different major risk variables : ETS on the one hand, socioeconomic status and paraoccupational exposure on the other.